Geographical variations in cancer incidence can be attributed to environmental causes in general, and to diet, in particular. We are concerned that the reason why African Americans (AAs) have the highest incidence (c1:70,000 of the population) and death rate from colon cancer in the USA is due to dietary factors as native Africans, who consume a very different diet, hardly ever get the disease (<1: 100,000). Studies of ours in the 2 communities, supported by a wealth of experimental evidence, have suggested that the explanation may lie in the high dietary intake of red meat by AAs which increases microbiota populations of sulfur-reducing bacteria (SRBs), which produce cytotoxic and genotoxic hydrogen sulfide as a terminal product. This leads has been shown experimentally to lead to chronic mucosal inflammation and hyperproliferation, a state that increases cancer risk. In contrast, Africans are protected by high populations of methanogens which thrive in high carbohydrate, meat-free conditions and produce the non-toxic terminal product, methane. Our studies have also showed higher populations of secondary bile salt producing bacteria in AAs which are stimulated by high animal fat diets to produce carcinogenic secondary bile salts. In contrast, a high resistant starch diet stimulates mucosal-protective Lactobacillus species were, indeed, found to be more common in Africans. These observations have lead to our hypothesis that the risk of developing cancer of the colon is determined by the interaction between diet and resident microbiota, which influences the level of chronic inflammation and epithelial proliferation - and therefore cancer risk - in the colonic mucosa. In the present proposal, we plan to substantiate this hypothesis by studying 20 healthy middle aged subjects from the population of AAs in the Pittsburgh area and compare them to the same number of Africans before and 2 weeks after dietary switch. Specifically, we will change the AA diet to a high resistant starch, low meat diet and the African diet to a high red meat, low carbohydrate westernized diet. If our hypothesis is supported, we will expect to observe in the AA group an increase in methanogenesis and a reduction in colonic SRBs and hydrogen sulfide production, resulting in a reduction in mucosal inflammation and colonic epithelial proliferation, our primary biomarker of cancer risk. In contrast, we will expect to find increased SRB population growth in Africans with suppression of methanogenesis. Microarray analysis of mucosal gene expression, followed up by RT- PCR confirmation will be employed to identify novel pathways that may explain the mechanisms that link diet, bacterial metabolism, inflammation, and hyper-proliferation. Our findings will provide insight into how the diet can be manipulated to modify microbiota to promote mucosal health, and therefore reduce colon cancer risk, diminish health care costs related to diagnosis and treatment, and decrease the unacceptably high present morbidity and mortality from this disease in African Americans. PUBLIC HEALTH RELEVANCE: Colon cancer is the second leading cause of cancer death in the USA, and compared to other U.S. racial groups African-Americans have the highest incidence (70 per 100,000 of the population) and mortality. These statistics contrast sharply from those reported in native Africans where incidence rates are <1 per 100,000. Our previous studies have suggested the difference can be attributed to the relatively higher meat and animal fat intake by African Americans and the higher resistant starch intake by Africans.